Changing the order of items in a menu
From Joomla! Documentation
1. Log into the Joomla Administrator backend
2. Hover mouse over Menu
3. Select the Menu Set that the menu item is in that you wish to change order (see figure 1)
4. Once inside the Menu Set, i.e. My main*, menu order can be changed via the up/down green arrows or by actually typing the sort number. (see figure 2)
5. Alternatively, you can also go into the Menu Item and change order there. (see figure 3)